When the screw slips, the material may gather at the feeding port and cannot be conveyed to the end of the injection machine normally. Screw slip occurs in the plasticizing section when the screw rotates and retreats in the barrel to convey material and prepare for the next injection. At this time, the rotation of the screw continues, but the axial movement of the screw stops, that is, slip occurs.
Screw slippage often leads to material degradation before injection, product quality declines (such as material shortage), and molding cycle time is prolonged. There are many reasons for screw slippage, which may be related to excessive back pressure, overheating or undercooling at the end of the barrel, wear of the barrel or screw, too shallow thread in the feeding section, unreasonable design of the hopper and blocked hopper, wet resin, excessive resin It is related to factors such as lubrication, material too fine or unreasonable cutting of resin and recycled material.
Overcooling at the end of the conical screw barrel
is one of the main causes of screw slippage. The barrel of the injection machine is divided into 3 sections. At the end, that is, the feeding section, the pellets will form a layer of melt film and stick to the screw during the process of heating and compression. Without this film, the pellets cannot be easily transported to the front end.
The material in the feed section must be heated to a critical temperature to form that critical melt film. However, usually the residence time of the material in the feeding section is too short to reach the required temperature. And this usually happens on small injection machines. A residence time that is too short will result in incomplete melting and mixing of the polymer, resulting in screw slippage or stall.